Step 1: Preheat your oven to 350°F (175°C) and grease or line an 8x8-inch baking pan with parchment paper, leaving an overhang on the sides for easy removal.
Step 2: In a medium bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, ginger, and nutmeg until well combined.
Step 3: In a large bowl, whisk the granulated sugar, brown sugar, vegetable oil, eggs, and vanilla extract until smooth and well incorporated.
Step 4: Gradually add the dry ingredients to the wet ingredients, stirring gently until just combined. Do not overmix.
Step 5: Fold in the grated carrots and chopped walnuts (if using) using a spatula until evenly distributed throughout the batter.
Step 6: Pour the batter into the prepared baking pan and spread it evenly. Bake for 28-32 minutes, or until a toothpick inserted into the center comes out clean.
Step 7: Remove from the oven and allow the bars to cool completely in the pan on a wire rack.
Step 8: While the bars cool, prepare the glaze. Melt the butter in a small saucepan over medium heat. Stir in the maple syrup and whisk until combined. Remove from heat and sift in the powdered sugar, whisking until the glaze is smooth and glossy.
Step 9: Pour the maple glaze over the cooled carrot cake bars and spread evenly. Let the glaze set for at least 30 minutes before slicing into squares.